(1) Field of the Invention
This invention is directed to the field of amplifier circuits, and more particularly, to a differential sensor element preamplifier.
(2) Description of the Prior Art
When utilizing sensor elements to detect and measure sea noise, low noise circuits are desirable for measuring ambient sea noise levels. For many years, hydrophone preamplifiers with a single-ended input have been used for various applications because of the ease of obtaining the desired very low noise floor at the input. These amplifier circuits usually employ very low-noise FET's (Field Effect Transistors) to give an ultra-high input impedance that avoids generating current noise problems when terminated with an input resistor of many megohms. Such amplifier circuits have been used successfully.
A disadvantage with single-ended type sensors such as hydrophones is the susceptibility to pickup and amplification of capacitively-coupled noise such as that at sixty hertz. This problem becomes most obvious when using long hydrophone cables. Such input noise pickup degrades the signal-to-noise ratio in the most sensitive portion (lowest signal level) of the entire system. Often, the benefits of using low-noise devices are destroyed by capacitively-coupled input noise.
Differential-input amplifiers solve this problem because they cancel out common-mode noise at the amplifier input. In general, the use of differential-input amplifiers for hydrophones has been limited until now because of the difficulty in calibrating them. This problem has been solved, and reference may be had in this connection to co-pending U.S. patent application entitled "Balanced, Double-Sided Calibration Circuit For Differential Preamplifier" of the same inventive entity as herein and fully incorporated herein by reference.
Other past difficulties with differential amplifiers have been in obtaining a noise floor comparable to that of discrete FETs, as well as the problem of saturating the first stage of the differential amplifiers with cable strumming noise and noise from mechanical sources, such as motor, machinery and tool transients. It has been difficult to provide precision balanced filtering at the very front end that would attenuate these low-frequency noise sources while maintaining common-mode input rejection. The first stage is especially vulnerable to saturation because it commonly has high gain to preserve the noise floor.
One prior art hydrophone preamplifier with self-calibration disclosed in U.S. Pat. No. 4,689,578 to Spychalski, includes two charge-coupled amplifier stages forming a differential preamplifier, two first order high pass filters providing low frequency roll off and a fully differential output driver. A pair of back-to-back diodes is connected directly across the hydrophone.
In the device disclosed by Spychalski, diodes regulate the current to the charge-coupled amplifiers. If, however, the hydrophone charges to too high a level, the unprotected diodes can fail causing the differential preamplifier to become saturated or damaged due to the resulting voltage spike.
In the Spychalski device, the roll off frequency of the two first order high pass filters is adjustable by a frequency control signal that changes the time constants of the filters. The gain of the amplifiers can be set by feedback capacitors. The gain of the preamplifier can be set by a gain change signal. The preamplifier is balanced for common-mode noise rejection by a trim capacitor. For each setting, and for each change in the settings of the gain and of the roll off frequency, the preamplifier needs to be checked for unbalance. Not only is this disadvantageous, insofar as each different setting requires another re-balance process, but more importantly, it is inherently labor-intensive to set and to maintain precision balance by means of varying the value of a trim capacitor. Trim capacitors can also age and drift with time and temperature excursions, necessitating frequent re-adjustment.